Add comment and prevent IndexOutOfBounds exception in ImageResponse

[devholic22]

Expected Behavior

Previously, an IndexOutOfBounds exception might occur if the developer using the getResult function, I modified returns null. (if imageGenerations is empty)

Current Behavior

In this version, the getResult function returns index element number 0 of imageGenerations directly.

Context

• I added comments to make it easier to understand. (I referred to ChatResponse.)
• I modified to return null if imageGenerations is empty.
• I modified toString like ChatResponse format.

please see #745